Many documents are known describing processes for the catalytic conversion of (gaseous) hydrocarbonaceous feedstocks, especially methane, natural gas and/or associated gas, into liquid products, especially methanol and liquid hydrocarbons, particularly paraffinic hydrocarbons. In this respect often reference is made to remote locations and/or off-shore locations, where no direct use of the gas is possible. Transportation of the gas, e.g. through a pipeline or in the form of liquefied natural gas, is not always practical. This holds even more in the case of relatively small gas production rates and/or fields. Reinjection of gas will add to the costs of oil production, and may, in the case of associated gas, result in undesired effects on the crude oil production. Burning of associated gas has become an undesired option in view of depletion of hydrocarbon sources and air pollution.
The Fischer Tropsch process can be used for the conversion of hydrocarbonaceous feed stocks into liquid and/or solid hydrocarbons. Generally, the feed stock (e.g. natural gas, associated gas and/or coal-bed methane, coal, biomass, as well as residual (crude) oil fractions) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas). The synthesis gas is then fed into a reactor where it is converted in one or more steps over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight compounds comprising up to 200 carbon atoms, or, under particular circumstances, even more.
Numerous types of reactor systems have been developed for carrying out the Fischer Tropsch reaction. For example, Fischer Tropsch reactor systems include fixed bed reactors, especially multi-tubular fixed bed reactors, fluidised bed reactors, such as entrained fluidised bed reactors and fixed fluidised bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebullating bed reactors.
The Fischer Tropsch reaction is very exothermic and temperature sensitive, with the result that careful temperature control is required to maintain optimum operation conditions and desired hydrocarbon product selectivity. Indeed, close temperature control and operation throughout the reactor are major objectives.
Starting up such a process will involve new and regenerated catalyst material. However, catalyst material when new or regenerated is often more active than once it has achieved a steady state activity under reaction conditions. In chemical reactions such as the Fischer Tropsch reaction, which is very exothermic and temperature sensitive as mentioned above, a higher level of activity of a catalyst at the start up of a reactor is of significant concern. In a Fischer-Tropsch reaction, the higher activity can easily result in over-conversion that may result in undesired catalyst de-activation, for example due to higher water production or due to carbonation of the catalyst as a result of a decreased hydrogen-to-carbon monoxide ratio in the synthesis gas.
There is thus required a way of using the initial greater activity of new catalyst material until the reaction process reaches a steady state. Several start-up procedures have been proposed in the prior art to cope with the initial greater activity of the catalyst.
In WO 2005/026292 and WO 2005/026293, for example is disclosed a method for start-up of a hydrocarbon synthesis process in a slurry bubble column. The start-up method comprises a specific procedure for charging the catalyst particles in the conversion reactor. At the end of the charging phase, the reactor is continuously fed with inert gas to prevent catalyst sedimentation. During a subsequent conditioning phase, the temperature is brought to values suitable for conditioning, the inert gas is gradually substituted by synthesis gas up to a concentration ranging from 5-50 vol % and this concentration is maintained for 24-72 hours. Then, the pressure and temperature are gradually increased up to steady state regime values and the concentration of inert gas gradually reduced to zero.
In WO03/068715 is disclosed a process for starting up a Fischer-Tropsch reactor wherein synthesis gas is initially fed to the reactor at a flow rate below the steady state flow rate and having a H2/CO molar ratio above the steady state ratio. The synthesis gas flow rate is then increased and the H2/CO molar ratio in the synthesis gas decreased to the steady state values.
In U.S. Pat. No. 2,602,810 is disclosed a Fischer Tropsch process using, under steady state conditions, a reactor feed stream with a very high H2/CO molar ratio, i.e. at least 15, by combining synthesis gas with a hydrogen-rich recycle stream. The reactor is started by pressuring it with hydrogen, and then starting recycling. The reactor is then brought to a temperature required for initiation of the conversion reaction. Synthesis gas is then fed to the reactor at a low flow rate and hydrogen at a high flow rate. During start-up, the flow rate of synthesis gas is increased while decreasing the flow rate of the hydrogen.